The ARP 3620 keyboard is duophonic from a single resistor chain - It derives the upper voice by detecting the voltage difference between the two notes played...

Nice, i think i need to look into that circuit, sounds interesting.
Always good to learn something new._________________In an infinite universe one might very well
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My problem is solved - just scored two 5 octave Kimber Allen keyboard assemblies complete with double pole key blocks Both units in very good condition (just a few gold wires to realign), pulled from a 70's organ.

hey andy
would you be willing to share your work on your 3620 keyboard ?
I am thinking of doing a keyboard for myself to add to zthee's TTSH (nearly released project) . I might be able to nut it out for myself given a bunch of time but i believe you have a board already laid out i think ?

cheers
Denis_________________In an infinite universe one might very well
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hey andy
would you be willing to share your work on your 3620 keyboard ?
I am thinking of doing a keyboard for myself to add to zthee's TTSH (nearly released project) . I might be able to nut it out for myself given a bunch of time but i believe you have a board already laid out i think ?

cheers
Denis

The original keyboard I built was just a MIDI controller going through a CV converter, with various circuits to make it "look" like the 3620. Unfortunately there's no single/multi Trigger option and it's not duophonic (but it does have a simple PIC step sequencer )

I've now got a high quality version of the 3620 service manual from Musicparts and I've built the PCB, but I've got a bit of a problem with the upper voice memory... the CV just goes wild and is unusable at the moment. It's all a bit weird because the upper and lower cv memory circuits are the same, and the lower memory works fine Suspect it's to do with the S&H cap/FET/Gate.

glad you got it working andy and it looks great too boot.
with my home etched PCB's after i have drilled the holes, i print a component layout on normal paper using a bubble jet printer for a silk screen layer and stick it to the pcb top face with either board laquer or simply PVA glue never had a problem doing this , it does make stuffing the board so much easier and much less likely to miss a component .

the keybed i had hoped to use unfortunately only has the one buss.
With the 3620 circuit you need 2 seperate contact busses ?
couldnt see any way round that from the schematics
thinking i can add a row of micro switches underneath to get a second buss however , rather than source another keybed ,
also going for 3 octaves rather than 4 as the keybed in this case pyhsically cant be longer than 600mm so I will have to adjust the constant current source to suit._________________In an infinite universe one might very well
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Andy how did you go with the rare dual fet IMF3958 replacement
did it work out to be not required in the end ?
also i dont get the odd symbol - oval around the LF442 inputs, haven't seen that one before... grounded guard ring perhaps ?_________________In an infinite universe one might very well
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Andy how did you go with the rare dual fet IMF3958 replacement
did it work out to be not required in the end ?
also i dont get the odd symbol - oval around the LF442 inputs, haven't seen that one before... grounded guard ring perhaps ?

The 3620 needs a dual bus for duophonicity (!!! is there such a word !!!) - If you don't need two notes, take a look at the ARP Solus schematics... Single bus for both CV and Gate/Trigger (I tried Ray Wilson's single bus converter, but didn't get any joy from it)

I replaced the IMF with a couple of 2N5459's... seems to work ok, although I'm still pretty sure you could bypass the FETs and just use a JFet op Amp (but as it's all working well, I'll leave things alone)

Yes, the oval shape indicates a guard ring around the FET and .47 Cap to help reduce leakage.

As it stands, using the 2N5459's and a standard box type .47 poly cap, the S&H drop is about 1 semitone over 20 mins - I can live with that

As it stands, using the 2N5459's and a standard box type .47 poly cap, the S&H drop is about 1 semitone over 20 mins - I can live with that

As can I, that is pretty damn good actually
I have 6 of rays single buss keyboard controllers floating around here
they are maybe half as good as that on average , but it is a much smaller
cap used for the S/H only 10nf, maybe some improvement could be had by running 2 x S/H in series one for fast acquisition and the second optimised for a long duration however this isn't so much about that, so i wont play around with it besides if i am holding a note for 20 minutes it just means i fell asleep.

Guard ring cool easy peasy._________________In an infinite universe one might very well
ask where the hell am I
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I am waiting for the PCB/case and parts to build a TTSH like many others, and really want a 3620 keyboard to go with it and would love to build my own along the lines of AndyR1960's.

I note the original 3604P can be converted to a 3620 according to the 2600 service manual so would intend to use one of those as a base. It's a 3-bus but would obviously be good as a 2-bus by ignoring one bus. Failing that a new Kimber Allen keybed would do but at what cost I wonder??? I'm not too far from them as it happens.

AndyR1960 - any chance of a parts list, board and panel layout in an appropriate format (PDF)? I can do PCBs, woodwork and metalwork so quite up for the challenge.

Looks like you've used a slide switch instead of the 3-way rocker - would an NKK M2044TNW01EB DP3T do the trick? It's quite faithful to the old ARP rocker/paddle design.

Regarding the IMF3958, Littlediode are advertising them at fairly horrendous cost, but if that's what it needs ideally for stability, then I'd go with that.

BTW ARP's Field Change Note FCN005 suggests replacing the 2N5459 FETs with 2N4392 if you hadn't seen it.

Regarding the IMF3958, Littlediode are advertising them at fairly horrendous cost, but if that's what it needs ideally for stability, then I'd go with that.

UTsource list them for $6:00 each so about a quarter of littlediodes price but of course you risk getting fakes
having said that they have proven good for me in the past with other rare parts._________________In an infinite universe one might very well
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For those who may be interested in building a 3620 keyboard clone, and are fed up with trying to find a suitable donor keyboard/organ (without having to fetch the whole damn thing home only to have to break it up and dispose of most of it), I have been in touch with Kimber Allen in the UK and they have a "limited quantity" of 37 and 49-key Italian S.K.A. keybeds on which they can fit two independent gold contact sets to each key, for £296 +VAT and delivery (49-key price). From the photo they've sent me it looks just the job.

I'm getting all the documentation together for the Duophonic keyboard… It's all a bit messy at the moment because I don't take notes while I'm building (I know… cardinal sin, but I tend to work on the fly, and forget to write things down).

I knew about replacing the 2N5459 with the 4392, but I didn't have any of them around, so I kept the 5459 in… no problems with it

The KA keybed pic with the DP contacts is correct… although I got lucky on ebay with an organ builder who gave up on his project, but I had to disassemble all the contact blocks, clean up the gold wires, straighten them and rebuild them!

Thanks for the update Andy. This thread's been quite quiet recently and I seem to have woken it up.

I've compiled most of the parts list (all but switches and caps) for an exact 3620 clone taken straight from the schematic diagram (the high-res scan version purchased from Music Part), and have an original ARP Service Manual on its way to me just to check there's no wires that haven't scanned fully and would hence get missed. Try as I might I could not find R18 on the schematic anywhere - did you? Maybe it was deleted from the original design or simply missed during the component numbering??

I like the idea of the LED sliders to match my soon to be started TTSH (no disrespect to your fantastic achievements of course) so I have used the Bourns part numbers for those but haven't so far worked out the LED supply- I expect a simple resistor dropper from the +15V power rail will suffice and run the three LEDs in series.

I'm struggling to find the right matching slide/toggle switches at the moment though. Only S3 (repeat switch) seems to really need a DP3T as S1 (octave transpose) can be a simple DPDT or even SPDT with centre-off, but for authenticity I'm going to use an NKK paddle switch such as M2044TNW01-EA. For S3 a Switchcraft 46313LDRX should be OK but it's solder tag only, so for S2 and S3 I'll need to find something of similar proportions.

for solder tag switches i generally adjust hole and pad sizes to make them fit a PCB nicely, just create it as a custom component to put in the library.
matching mechanical heights are the only concern really._________________In an infinite universe one might very well
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Well I'm getting on quite well with my 3620 clone design and just laying out the PCB and front panel. Nothing is built yet but I have everything I need, apart from time of course.

I've gone for an exact component for component match with the original circuit (with the Q1 and Q4 modification), except for orange LED sliders to match the TTSH and modern switches and sockets, all PCB mounted on a double sided board.

One minor point is troubling me at the moment before I commit the board to manufacture (don't want an expensive mistake) is the link (if there is one) between the LFO delayed sine output and the external vibrato input.

The original front panel design shows an arrow between the two sockets suggesting that they are perhaps linked by the switched contacts of one or both sockets so it feeds back into the external vibrato input if no plugs are inserted in either socket. The service manual does not show any connection between the two on the schematic nor mention it in the text. It's a simple job to connect them if required.

I also presume the Lower Voice Output (CV) goes via the lead to the 2600/TTSH as the main CV output, and the Upper Voice Output goes to both Upper Voice Output sockets on the front panel.

Does anyone know if that is correct or not? I don't have access to a real 3620 to check the internal wiring and the only internal photo I've ever seen is of no help.

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